How to get the missing library and its collection with Python?
Look at the library filepaths.
Iterate over the libraries, for any that do not exist in saved filepath, convert the filepath and reload.
import bpy
from pathlib import Path

for lib in bpy.data.libraries:
    p = Path(bpy.path.abspath(lib.filepath))
    if not p.exists():
        
        print(&quot;Missing:&quot;, lib.filepath)
        # convert filepath 
        lib.filepath = &quot;/some/converted/path&quot;
        # with new valid path
        lib.reload()